A) Field of the Invention
The present invention relates to an optical semiconductor device having a ridge type waveguide with an active layer containing a p-type region being formed on a p-type cladding layer, the optical semiconductor device being applied to a semiconductor laser diode, an optical amplifier, an optical gate switch and the like for optical fiber communications.
B) Description of the Related Art
Presently, a quantum well structure is mainly adopted for an active layer of a semiconductor laser diode used as a light source of optical communications. Improving performance of a semiconductor laser device by adopting a quantum dot structure to an active layer has recently been researched. As the quantum dot structure is adopted for an active layer, a temperature dependence of laser characteristics can be mitigated considerably.
Description will be made on a semiconductor laser diode disclosed in the study by T. Yamabana et al, “Temperature independent transmission for 10 Gbps 300 m-MMF using low driving-current quantum dot laser”, OFC/NFOEC 2006 OFL3. An active layer, which includes barrier layers and quantum dot layers stacked alternately, and a p-type AlGaAs cladding layer are formed on an n-type AlGaAs cladding layer. The barrier layer constituting the active layer is made of GaAs partially doped with p-type dopant. In the following, the active layer of this type is called a “p-type active layer”. Current-light output characteristics of this semiconductor laser diode are least dependent upon temperature near a room temperature. Under the condition in which the drive conditions are fixed, high speed operation about 10 Gbps and multi mode fiber (MMF) transmission are realized in a temperature range between 20° C. and 80° C.
FIG. 4A is a schematic diagram of a ridge-type semiconductor laser diode adopting a p-type active layer using an n-type substrate. A p-type active layer 101 is formed on the whole surface of an n-type substrate 100, and a ridge-shaped p-type cladding layer 102 is formed on a partial surface of the p-type active layer. This laser diode has a large capacitance because the interface between the n-type substrate 100 and p-type active layer 101 extends over the whole device area. Therefore, this laser diode is difficult to operate at high speed like 10 Gbps.
FIG. 4B is a schematic diagram showing a semiconductor laser diode disclosed in the above-cited document. In this example, a p-type active layer 101 is also patterned as a p-type cladding layer 102, and constitutes a portion of the ridge. This structure is called a “high mesa structure”. As the high mesa structure is adopted, an increase in device capacitance can be prevented because the pn junction does not extend over the whole device area and is restricted only in the ridge portion. However, with the high mesa structure, the active layer 101 itself is etched and damages during etching are left in the active layer 101. It is therefore difficult to retain reliability sufficient for practical use.